4.5 Rehabilitation Strategies for Pastoral/Arable Production Systems
4.5.1 >300mm
The devastation from 300mm of ash or more would be so great that the land would be rendered unfarmable for many generations and rehabilitation would be at an extreme cost. Bringing this land back into intensive agricultural production in the medium term (20 - 40 years) is unlikely to be economic. In the long term, alternative land uses would need to be explored, such as forestry.
The immediate requirement on land affected to this degree would be relocation of the affected residents and possible adjustment programmes to enable people to re-establish themselves elsewhere.
4.5.2 100 mm - 300 mm
Rehabilitation of the land with 100-300mm of ash would be extremely difficult, as the ash layer would have sterilised the soil below and would be too deep to be incorporated with conventional cultivation techniques - ploughing, discing, rotatilling etc. Rehabilitation on hill country would be dependent on the rate of erosion of the ash and the fertiliser and seed inputs.
Restoration of land is dependent on removal of the ash layer or a much longer time frame of recolonisation of the ash layer as organic matter and fertility builds up for plants to survive.
The time frame for restoration of the land back for pastoral farming could take generations. Initially species adapted to the harsh environment need to be established to start building up organic matter to support more productive species. The type of plant species suitable to recolonise the ash will be dependent on the climate. As the tephra has low permeability it is more prone to drought. This combined with the lack of fertility make species like Lupins and Lotus species (that fix nitrogen) and possibly the Marran grasses as possible recolonising agents.
The process of re-establishing vegetation is likened to that of plant invasion and colonisation of sand dunes (RJ Cook et al, 1981).
Initially, just restoring basic facilities such as roading, water supply, power, effluent systems would be required before restoration of the land could be considered.
Farming the land as it was known before the eruption may not be an option, with a long term view to be taken in any restoration programme.
As with 300mm of ash or greater, relocation of affected residents and
associated financial resources, compensation, counselling etc may be required for rural
communities to re-establish themselves.
4.5.3 50 - 100 mm
Rehabilitation of the land will be influenced by contour, availability of suitable machinery, finance and human resources.
As the ash does not dissolve or percolate into the soil profile the ash needs to be incorporated with high inputs of fertiliser to provide a suitable medium for establishing ryegrass/white clover pastures. On land able to be cultivated, deep ploughing (to 20 cm) would give the best results as the ash layer would be buried.
Even with the incorporation of the ash, the soil characteristics change with:
- Reduced evaporation
- Lower fertility
- Lower permeability of the soil.
Any large scale cultivation would be expensive. Costs include regrassing, fertiliser and high machinery costs due to the abrasive nature of the ash increasing wear and tear.
Rehabilitation of land affected by ash is similar to development of sand country where the initial requirements are establishment of any species tolerant of the conditions to stabilise the ash and build up fertility.
Re-establishment of pastoral species is also dependent on the nature of the ash. Where it was very acidic, liming maybe required, along with high fertiliser inputs to create a soil medium, conducive to pastoral growth. Initially, acid tolerant species may need to be planted and species more tolerant of severe conditions, i.e. Marram grass, lupins, Yorkshire Fog and Lotus. These species tend to be lower yielding than the existing ryegrass and clover pastures. Once soil fertility and organic matter levels increase, more productive species may be established.
Where land cannot be cultivated, rehabilitation will be a slow process. Oversowing of low fertility species with fertiliser inputs may be required. It may be uneconomic to bring some land back into pastoral production and a change in land use may be the best solution.
Any rehabilitation programme will be very dependent on the financial resources of the farmer. With an ash shower of up to 100 mm, the short term impacts could be so severe on the financial viability of the business it is unlikely that the farmer will have the financial resources to stay on the land let alone implement a recovery programme. An eruption in winter/early spring would have the most devastating impact on cashflow.
Even a 100 mm ash fall may render some land uneconomic to be returned to it's original land use and alternative land uses such as forestry may be the best option.
As an ash shower of up to 100 mm may require de-stocking of the land for at least 6 months (until pastures can be re-established) any rehabilitation programme will require restocking. This may not be physically possible where the eruption devastated a large part of the country e.g. Taupo volcanic scenarios. The slaughtering of stock in the short term will result in a loss of valuable stock of high genetic merit. This will take many years to rebuild.
Until the ash settles, quality water for stock will be scarce, particularly when sourced from streams, rivers and dams. Extra expense will be incurred in maintaining water pumps as these are affected by the abrasive nature of the ash.
Ash will cause short circuits in electrical transformers and insulators although power lines are unlikely to collapse from the ash. There is greater danger from ash laden trees falling on power lines causing power failure.
Physical removal of the ash from buildings, yards, roadways etc will be difficult due to the large quantity. Where the ash is removed, dumping sites would be required (e.g. old quarries and existing land fills). To minimise wind reworking the ash dumps the surfaces of the disposal sites need to be stabilised with top soil and seeded with grass (Tilling et al 1990).
4.5.4 25 - 50 mm
Rehabilitation of the land will be greatly influenced by time of year of the ash fall and the nature of the ash. The most critical period for both dairy and sheep and beef would be the late winter early spring period when pasture covers are low and supplementary feed has largely been used up.
It will also be difficult to procure supplements from other areas or sufficient grazing to de-stock affected farms at this time of the year. Management options would be to mob stock up and move them through longer pasture areas of the farm to shake/remove ash off the plants. These areas could then be grazed with some supplementary feeding of bought in feed eg, meal, hay if available etc.
Other options would be to de-stock the farm as much as possible by sending stock to the works or for grazing in other parts of the country. This may involve moving stock a considerable distance eg from Central North Is to the South Is. The logistics of moving large numbers of stock is not considered feasible.
Paddocks that had been intended for cropping or pasture renewal could still be cultivated and sown in fast growing annual crops such as some of the annual rye grasses, feed oats and barley. Soil fertility is likely to decrease, requiring higher fertiliser inputs.
If the ash shower occurred in the summer/autumn period, management options would be eased by having the ability to de-stock lambs, prime cattle and cull dairy cows to the works before too much value was lost; reserves of hay/silage would be at their greatest and for those who traditionally put in a greenfeed crop such as maize, choumollier or swedes, these would be of sufficient maturity to supply a substantial amount of feed.
No matter when the ash shower occurs the provision of good quality water would be essential. Under most dairying situations this would require some increased water pump maintenance and cleaning of troughs once ash falls have ceased as most dairy farms would be on deep well bores and reticulated systems.
On farms (more likely to be sheep/beef/deer) taking supplies from streams or dams, provision of good quality water for both human and stock consumption would be more difficult. Outside assistance may be required in the short term i.e. until streams clear and dam water can be tested clear of toxic chemicals.
As with the 50-100 mm scenario rehabilitation of any farm will be dependent on the financial resources of the farmer and the robustness of the farm business as even a 50mm fall of ash will have serious financial implications in the year of the ash fall and following season due to lost production and increased costs.
4.5.5 5-25 mm
Rehabilitation of the land would follow similar lines to the 25-50mm scenarios, where pastures were destroyed through the burning of ash. Where the ash did not burn or scorch the pastures, rehabilitation would be similar to that after a severe dry period. Pastures would need to be re-established either through conventional cultivation or undersowing. The method chosen would be dependent on the ash depth. Where the ash was up to 25 mm deep, incorporation of the ash through ploughing would be best to return the land quickly to its pre-eruption productive value.
Rehabilitation of hill country would be slower as the material cannot be incorporated into the soil profile. Rainfall will improve the rate of recovery as the ash is eroded. Oversowing with fertiliser will be necessary where the pastures are weakened/destroyed by the ash and because of the inherent lower fertility of the soils.
Extra supplements would be required to maintain stock numbers until pastures recovered. Greenfeed crops and high producing annual ryegrasses could be established where the eruption occurred late summer, to provide increased feed in the winter until permanent pastures could be established.
Maintaining farm operations would be important after the eruption, especially providing good quality water and maintaining farm machinery.
Farms with high debt loading would struggle to meet their debt servicing commitments, due to reduced production in the short term (12-24 months) and the costs of re-establishing pastures after the eruption, the effects being similar to a severe drought.
4.5.6 0-5 mm
With the impacts of ash up to 5 mm being negligible, rehabilitation of the land would not be necessary. Increased supplementary feed may be required, while the stock were put off their feed. This should be a short term measure until the ash is washed and consolidates to 1-2mm.
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